MEIOSIS AND I-LRTILITY IN POLYPLOIDS 



convcrgently. Or, like two bivalents, they can lie parallel in a 

 rectangle. Again, middle members of these configurations can lag and 

 be lost so that separation may be two-and-two or three-and-one 

 or two-and-onc with one lost. The products of meiosis are thus 

 sometimes equal in their numbers of chromosomes, but perhaps 

 equally often they show an error of one or two above or below. 

 This error is what reduces the fertility of tetraploids, but the 

 reduction is o£ the order of one quarter or less, not as with triploids 

 ot the order of one half, for each chromosome of the basic set. 



We learn several new principles from these observations. Li the 

 first place, we see that the triploid is a numerical hybrid. It has two 

 nuclear units from one side and only one from the other. Like other 

 hybrids or heterozygotes it is incapable of breeding true and even 

 its loss of fertility, as we shall find later, is characteristic of hybrids 

 which arise from crossing species. 



The genetic system of the triploid is adjusted until it comes to the 

 production of germ cells. It then breaks down. It does so because 

 at meiosis new individuals, new self-propagating systems, with new 

 and untried combinations of chromosomes are brought into being. 

 Unbalance, which appears in mature plants and animals as the cause 

 of abnormal forms, appears in immature individuals as the cause 

 of death. The fertility of the parent is the inverse measure of the 

 frequency of death of the offspring. The segregation of differences at 

 meiosis is thus a prime cause of sterilitv. 



Secondly, we see that development follows a standard course in 

 organisms with the standard set of chromosomes equally multiplied. 

 But where the multiplication is unbalanced, development is 

 unbalanced and disturbed. Evidently all the cliromosomes are 

 different in their effects on growth, just as they and their parts are 

 different in their specific attractions for one another at meiosis. 

 This disturbance accounts for the shghtly reduced fertility of 

 trisomies and of tetraploids, whose segregation is slightly irre^^ular, 

 and for the greatly reduced fertihty of triploids, whose segregation 

 is grossly irregular. 



An exception proves this comiexion between balance and 

 fertility. In triploid hyacinths, HyaciiitJms orientalis, fertihty is not 

 greatly reduced, and plants with all intermediate numbers between 

 diploid and tetraploid appear in their progeny. The development 



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